A social network based on the same birthday. There are only 365 days a year so the network of people with the same birthday is growing every year. There are about 23k birthdays in NYC on any given day. An app that you download on your birthday to party with other fellow birthday friends and see where people are congregating.
Businesses can give discounts to birthday goers.
Also a dating app which would make your partner’s birthday much easier to remember.